# Need a voltage gain of 1000!

#### S_lannan

Joined Jun 20, 2007
246
The reason you may be getting less gain is that most signal generators have an impedance of 50ohms. A common base amplifier generally has an impedance of 20 - 50 ohms. You may be losing signal at the input of the amplifier.

I would probe the amplifier to see what actually makes it to the input and see if the individual stages offer enough gain.

Once you find what isn't working to specification, you can modify it.

#### gusmas

Joined Sep 27, 2008
239
ok here is my graphs .... and circuit

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#### The Electrician

Joined Oct 9, 2007
2,778
Could you also show a probe of the collector of the first transistor?

What transistor model did you use? Is it a model of a particular transistor built in to the simulator?

#### S_lannan

Joined Jun 20, 2007
246
at 160hZ those coupling capacitors are too small....

#### Wendy

Joined Mar 24, 2008
22,145
Reactance = 1 / 2 pi F C = 1 / (6.28 10e-6 160) = 99.4Ω

I don't think their too bad, but you are loosing under half your total input to reactance. I'll have to study the design a bit myself. Still fighting the computer...

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Rethinking that, try upping all your capacitors to 1000 µF. This really will put them into the realm of negligable values.

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#### S_lannan

Joined Jun 20, 2007
246
A point I forgot to put across was in his circuit he has set the Q point for the common base stage at 7mA. This makes Hib ('re) around 3.6 ohms. The coupling cap has a reactance (Xc) of 10 ohms at 160hz. In this case there are significant losses. Either increase the coupling cap size or consider changing the operating point of the transistor to increase Hib.

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